Install Home Assistant Core failure when running hass

Having given up on VB I’ve had a crack at doing an “Install HomeAssistant Core” on Debian 10 Buster. Everyting looked fine until I get to the last command, which is to run hass. The errors I get are:

(homeassistant) [email protected]:/srv/homeassistant$ hass
Traceback (most recent call last):
  File "/srv/homeassistant/bin/hass", line 10, in <module> sys.exit(main())
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/__main__.py", line 278, in main args = get_arguments()
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/__main__.py", line 60, in get_arguments
    import homeassistant.config as config_util
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/config.py", line 52, in <module>
    import homeassistant.helpers.config_validation as cv
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/helpers/config_validation.py", line 85, in <module>
    from homeassistant.helpers import (
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/helpers/script_variables.py", line 6, in <module>
    from . import template
  File "/srv/homeassistant/lib/python3.7/site-packages/homeassistant/helpers/template.py", line 19, in <module>
    from jinja2 import contextfilter, contextfunction
ImportError: cannot import name 'contextfilter' from 'jinja2' (/srv/homeassistant/lib/python3.7/site-packages/jinja2/__init__.py)

So no help from anyone?
Not being able to turn on our porch lights is getting be a big deal this time of year :frowning: